The friendliest place on the web for anyone with an interest in aquariums or fish keeping!
If you have answers, please help by responding to the unanswered posts.
I think 12 would be a good figure, depending on your filtration 20 might be a bit of a strain. Only you will know, it is your tank and you will be caring for them. You could add a few and if you have no problems add a few more later.